The invention relates to the field of optical information storage and more specifically to optics systems for scanning rotating optical disks.
The invention relates to an optical scanning device for scanning an optical record carrier with an information layer, the device having an objective lens for converging a first radiation beam to a spot on the information layer.
An optical scanning device of the this type is known from the article xe2x80x9cTilt correction in an optical systemxe2x80x9d by Gerber and Mansuripur, Applied Optics, Vol. 35, No. 35, pp. 7000-7007. In the known device a radiation source generates a radiation beam, which is converged to a spot on an information layer by an objective lens. The radiation beam reflected from the record carrier falls on a detection system. The electric output signals of the detection system are used to form a tilt signal, representing the tilt between the normal to the record carrier and the optical axis of the objective system. The tilt causes a comatic aberration of the spot. The tilt signal is used to control a tilt corrector arranged in the path of the first beam to compensate the comatic aberration. It is a disadvantage of the known scanning device that the quality of the spot is not sufficient when scanning high-density optical record carriers.
Those skilled in the art are also directed to U.S. Pat. No. 4,224,533.
The above references are hereby incorporated herein in whole by reference.
It is an object of the invention to provide a scanning device with a high-quality spot that is sufficient for scanning high-density optical disc carriers such as DVD disks.
The object is achieved in accordance with the invention by a scanning device as described in the opening paragraph, that includes a first and a second detection system for receiving a second radiation beam from the record carrier and arranged before and after an image of the spot, respectively, the detection systems being adapted for determining an intensity profile of incident radiation. The scanning device also includes an electronic circuit connected to electric outputs of the first and second detector for forming an electric signal representing a wavefront aberration of the second radiation beam.
The invention is based on the insight that the optical aberrations in the second beam may be determined from a measurement of the intensity profile of the radiation beam at a position before the image of the spot and at a position after the image of the spot. The value of an optical aberration can be determined from a combination of intensity values of both detection systems. If the second beam is split into two branches and the first and second detection system are arranged in the first and second branch, respectively, both detection systems should receive radiation from the same cross-section of the second beam to ensure proper determination of the wavefront aberration. The measured values of the aberrations allow control of optical elements in the path of the radiation beam that compensate for the effect of the aberrations, thereby improving the quality of the spot formed by the beam on the record carrier. The optical aberrations are the primary aberrations, such as spherical aberration, coma and astigmatism, and higher-order aberrations. It should be noted that optical aberrations do not include defocus.
A special embodiment of the detection system includes a central detector, preferably arranged on the optical axis of the second beam, and an annular detector arranged around the central detector. Such a detection system is very suitable for the detection of spherical aberration in the second beam and also allows determination of the focus error.
Another embodiment of the detection system includes three strip detectors. The detection system allows the determination of both the focus error and spherical aberration. It is also relatively insensitive to changes in the wavelength of the radiation. Such changes affect the path of the radiation beams, in particular where gratings or holograms are used.
It is noted that an optical scanning device having two detection systems, one before and one after an image of the spot, is known from U.S. Pat. No. 4,724,533. The electric detection signals of the first and second detection systems are used for forming a focus error signal, representing the axial distance between the focus of the first radiation beam and the position of the information layer. The detection signals are not used for forming signals representing optical aberrations in the second beam.
The objects, advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings, in which